This invention relates generally to solar cells and solar cell assemblies which are especially useful in space, and more particularly to a method for testing such solar cell assemblies to identify those that have an anomalously high susceptibility to ultraviolet degradation.
Solar panels are conventionally used as a source of electrical power for spacecraft such as satellites. The solar panels typically used for spacecraft include a substrate and a plurality of individual photovoltaic solar cells which are secured to a face surface of the substrate. The individual solar cells are electrically connected together to form a series-parallel solar cell array which, when oriented properly toward the sun, converts solar energy into electrical energy. A coverglass, typically made of a silica material and coated with an optically enhancing layer, covers the individual solar cells and together with the cells forms a solar cell assembly.
The efficiency of a solar cell is directly related to the amount of useful light which is absorbed by the photovoltaic component of solar cell. Only a portion of the light striking the top surface of a solar cell is useful. Another portion of the light striking the cell is non-useful; i.e., the light has wavelengths outside the range that is converted by the cell to electrical power; and yet another portion of light is reflected by the solar cell. To reduce the problem of light reflection, solar cells may employ an antireflective coating on the surface of the coverglass through which light enters. The coating may also serve to reflect non-useful wavelengths of light in order to minimize heating effects.
One of the most important considerations for solar assemblies and panels used on spacecraft is efficiency. If a solar panel degrades in the orbital space environment, it is difficult, if not impossible, to correct or compensate for the resulting loss of electrical power with the result that the useful life of the entire spacecraft is often prematurely ended.
In the deployed configuration, the solar panels are subjected to substantial thermal stresses; the solar cells and the front surfaces of the substrates and coverglasses are subjected to the intense heat of the sun while the back surfaces of the substrates are subjected to the extreme cold of outer space. Furthermore, the coverglasses may be susceptible to degradation (visible darkening) upon exposure to ultraviolet radiation. Some degradation is expected to lead to a normally low, nominal, end-of-life loss in performance. Abnormally high degradation, however, may be caused by defects such as impurities and/or contamination sites present in the coverglass and/or coating. The defects can absorb radiation at a particular ultraviolet wavelength or wavelengths and can result in visible darkening. This darkening of the coverglass or coating results in less useful light being transmitted to the solar cell material, which in turn lowers the efficiency and power generated by the solar cell. The darkening may also significantly contribute to an increase of the temperature of the assembly arising from the increased solar absorption by the darkened coverglass.
Conventional quality control methods for inspecting solar cell coverglasses examine properties such as trace element levels in the glass substrate and UV reflectance coatings, multilayer coating thickness, and optical quality. These measurements do not necessarily correlate with the propensity of a coverglass or coating to darken on-orbit. Further, these methods are susceptible to passing solar cell assemblies that initially meet quality specifications, but later visibly darken upon ultraviolet-induced degradation. It thus is desirable to provide a test method that identifies the susceptibility of the solar cell assemblies to darkening and degradation before putting them in orbit.
The aforementioned need in the prior art is met by the present invention which provides a method for determining the susceptibility of a solar cell coverglass assembly to degrade wherein the solar cell coverglass assembly is one of coated and uncoated coverglass. The method comprises the steps of (1) exposing the solar cell coverglass assembly to a pulsing laser source having an energy per unit area per pulse which is preselected to cause damage to a coverglass assembly having an anomalously high propensity to darken and leave undamaged a solar cell coverglass assembly having only a nominal, lower propensity to darken; (2) identifying damage caused by the laser; and, (3) characterizing the propensity of the coverglass to darkening as a function of damage.